Writing plugins

Plugins let you log or inspect reads and changes to reactive values. Signified calls your plugin's methods when these events happen.

Enable plugins

Hooks run while at least one plugin is registered with plugin_manager, as shown below. With no plugin registered, reactive values skip them.

Available hooks

Each hook receives the reactive object as its value argument:

Hook	When it runs
created	A reactive value is created.
read	Its current value is read.
updated	It sends a change notification.
named	It is given a name.

Create and register a plugin

Implement the hooks you need with @hookimpl, then register the instance. A plugin can also be a module of @hookimpl functions. This plugin counts new reactive values:

from typing import Any

from signified import Signal, Variable
from signified.plugins import hookimpl, plugin_manager

class CreationCounter:
    def __init__(self) -> None:
        self.count = 0

    @hookimpl
    def created(self, value: Variable[Any]) -> None:
        self.count += 1
        print(f"Created {value:d}")  # :d shows the object's type and ID.

plugin = CreationCounter()
plugin_manager.register(plugin)

x = Signal(1)
y = x + 1
print(plugin.count)  # 2

plugin_manager.unregister(plugin)

Use logging in place of print if your application already has logging set up. Unregister the plugin when you no longer need it. For a larger example, see the access tracker, which counts reads and writes for each reactive value.

When several plugins implement the same hook, the most recently registered one runs first. register raises an error if the plugin is already registered, if an @hookimpl method's name is not one of the hooks above, or if it does not accept a value argument.